All of these things were possibilities in the past (though some we didn't know about). However, in most sci-fi stories (where these doomsday scenarios are most often discussed), WW3, asteroids or viruses can be remedied by a change in humanity in some way the author thinks would be swell.
In the case of a gamma ray burst, though... Well, we could be living in a utopian society and still be obliterated. Douglass Adams was the only one who would touch something like that, and he's dead
On top of that, I think we, as a society, are becoming more interested with astrophysics (or Star Trek, Stargate, and Babylon 5, if you prefer), and as such like to hear/learn about these things. Combine that with global calamity selling, and you've got plenty of documentaries talking about how our solar system will move into a dark nebula and irradiate all of us to well done.
